1. Field of the Invention
The present invention relates to a method of manufacturing a polymer solid electrolyte that is useful for primary and secondary cells, and particularly to a cross-linked polymer solid electrolyte that is useful for film-like polymer cells. The present invention also relates to a cross-linked polymer solid electrolyte itself. Further, the present invention relates to an electrolyte for use in solid electrochemical elements such as primary cells, secondary cells, capacitors, electrochromic displays, and sensors. Moreover, the present invention relates to electrochemical elements employing the electrolyte, particularly thin solid cells.
2. Description of the Related Art
As solid electrolyte, inorganic materials such as .beta.-alumina, Li.sub.2 TiO.sub.3, RbAg.sub.4 I.sub.5, AgI, and tungstophosphoric acid have been developed and widely known. However, inorganic materials have drawbacks of 1) large specific gravity, 2) difficult in formation into a desired shape, 3) impossibility of obtaining soft and thin film, and 4) low ion conductivity at room temperature, which cause problems in practical use.
Recently, organic materials have become of interest as potential material that can mitigate the above-described drawbacks. Such polymer solid electrolyte is manufactured in a manner such that electrolyte (mainly organic salt), such as LiClO.sub.4 or LiBF.sub.4 --which serves as a carrier--is mixed and dissolved into a matrix polymer such as polyalkylene oxide, silicone rubber, fluororesin, or polyphosphazen. Such polymer solid electrolyte is lighter and more flexible than inorganic materials, and therefore can be easily machined or formed into film. Under such circumstance, active research and development have been conducted in recent years in order to obtain a polymer solid electrolyte that exhibits higher ionic conductivity while maintaining the above-described characters.
A presently known most effective method of imparting high ion conductivity is a technique in which aprotic organic electrolytic solution is absorbed into a polymer solid electrolyte in any manner in order to obtain solid electrolyte in the form of gel (see M. Armand, Solid States Ionics, 69, pp. 309-319 (1994)). Polymers that are usable as a matrix in the gel-type solid electrolyte are generally divided into 1) linear polymers such as polyether polymers and fluororesins, and 2) cross-linked polymers such as polyacrylic polymers.
Applications of the above-described linear polymers are shown in, for example, I. E. Kelly et al., J. Power Sources, 14, pp. 13 (1985) and U.S. Pat. No. 5,296,318. However, both cases have the problems of leakage of electrolytic solution from polymer and insufficient strength of film. Further, since electrolytic solution acts as a plasticizer for polymer serving as a matrix, the polymer itself dissolves into the electrolytic solution even when the temperature of the system increases slightly.
For the cross-linked polymers, there have proved a method in which a liquid monomer mixed with an electrolytic solution is polymerized to yield a cross-linked polymer including an electrolyte (see PCT/JP91/00362, International Laid-Open No. WO91/14294). However, in this method, when the crosslinking degree of the polymer is increased, the ion conductivity decreases considerably, and when the crosslinking degree is decreased, the solid strength (elastic modulus) of the polymer decreases, so that film having a sufficient strength cannot be obtained.
In Japanese Patent No. 1842047 (Invention A), the applicant of the present invention proposed a block-graft copolymer, which is a model of the present invention and the method therefor. Also, in Japanese Patent No. 1842048 (Invention B), the applicant of the present invention proposed a polymer solid electrolyte composed of a block-graft copolymer composition in which in order to increase the ion conductivity of the block-graft copolymer, there is mixed an inorganic salt containing at least one element selected from the group consisting of Li, Na, K, Cs, Ag, Cu, and Mg in an amount of 0.05-80 mol % with respect to the alkylene oxide side unit thereof.
In Japanese Patent Publication (kokoku) No. 5-74195 (Invention C), the applicant of the present invention proposed a Li cell which includes, as an electrolyte, a composite material composed of a Li ion salt and a block-graft copolymer similar to the above. Further, In Japanese Patent Application Laid-Open (kokai) No. 3-188151 (Invention D), the applicant of the present invention proposed a block-graft copolymer composition which is obtained by adding polyalkylene oxide to the above-described inorganic ion salt composite of a block-graft copolymer.
In the inventions B, C, and D, an organic solvent is added, together with an inorganic salt or the like, to a resultant block-graft copolymer in order to dissolve it, and after formation, the organic solvent is removed through drying in order to yield a polymer solid electrolyte. However, since all the polymer solid electrolytes is slightly low in terms of ion conductivity, they have not come into practical use.
In order to improve the ion conductivity, in Japanese Patent Application Laid-Open (kokai) No. 7-109321, the applicant of the present invention has proposed a composite solid electrolyte in which a nonaqueous electrolyte including a cyclic carbonate solvent and an inorganic salt is included in the same block-graft copolymer as that described above. Although the invention improved the ion conductivity, and at the same time increased the film strength drastically, it was found that if such a composite solid electrolyte is applied to household small cells in which characteristics at low temperatures (room temperature to -20.degree. C.) are regarded as important, satisfactory lower temperature characteristics are difficult to be obtained because of the high viscosity and the high melting point of the cyclic carboxylic acid ester. This problem necessitates adding, as a secondary component, a large amount of a low boiling-point linear ester or a carbonic acid ester, which is a generally known method for improving the low-temperature characteristics of cells. However, since these solvents are good solvents for the above-described block-graft copolymer, addition of a large amount of such a solvent causes dissolving of the polymer solid electrolyte itself.
Further, when the above-described composite solid electrolyte is applied to large-sized cells for use in electric vehicles and electrical power leveling systems and the like for operation at high temperatures (60-80.degree. C.), which cells are expected to come into practical use in the future, polyalkylene oxide having low vapor pressure is optimally used as a main component. However, even in this case, use of a large amount of polyalkylene oxide causes swelling and dissolving of the polymer solid electrolyte.
Meanwhile, electrochemical elements represented by cells have employed liquid electrolytes. Particularly, in recent years, electrochemical elements employing solid electrolytes free from the problem of liquid leakage have been intensively developed in view of safety. Among these electrochemical elements, those employing polymer solid electrolytes (hereinafter abbreviated as SPE) are expected to be developed. Since SPE is flexible as compared with inorganic solid electrolytes and enable free design of the shape of elements employing the same, there is particularly expected the development of new type cells having a reduced thickness, high energy density, and shape-related merits, which features are implemented by use of SPE and, as a negative electrode, a lithium metal or a carbon material or the like which occludes lithium.
The new type cells are expected to be applied to consumers' products such as cellular phones, notebook type personal computers, video recorders, other portable electronic devices, and power sources for IC cards. Also, in view of safety, large-sized lithium batteries employing SPE are highly expected to be used in electronic automobiles and load leveling equipment (Nature, 373, pp. 557-558 (1995)). Examples of SPE systems which have been studied include polyethylene oxide (PEO)-Li salts. Intensive studies have been carried out in an attempt to obtain block copolymers containing ethylene oxide (EO) from SPE systems serving as starting systems and to obtain EO-grafted polymers.
However, the ionic conductivity of these homopolymer electrolytes is insufficient, specifically not greater than 10.sup.-4 S/cm at room temperature, which is two orders of magnitude lower than that of electrolytic solutions used in currently commercialized lithium cells. Further, due to a large interfacial resistance present in the solid-solid interface between an electrolyte and an electrode, the internal resistance of an entire cell system becomes very large. Accordingly, application of SPE to solid cells has been impossible. To improve the ionic conductivity of SPE, there has been made an active attempt to obtain a gel electrolyte by curing an optically hardening monomer, an electron beam hardening monomer, or a thermosetting monomer, together with an organic electrolyte. As a result, an ionic conductivity of approximately 10.sup.-3 S/cm (at room temperature) has been attained. However, the strength of an electrolyte membrane is weak as compared with that of a conventional polymer electrolyte. Thus, cells employing the gel SPE have involved unstable cell characteristics due to the occurrence of an internal short circuit during the manufacture thereof.
Several attempts have been made to combine an electrically insulating material and an SPE component into a composite for the purpose of improving the membrane strength of the above-mentioned gel SPE. According to U.S. Pat. Nos. 5,102,752 and 5,378,558, a nonwoven fabric-SPE composite is obtained by impregnating a nonwoven aramid fabric with a solution obtained by dissolving a gel electrolyte component, and cooling the fabric or ultraviolet-curing the electrolyte through use of an epoxy cross-linking agent. According to Japanese Patent Application Laid-Open (kokai) No. 5-205515, a separator-SPE composite is obtained by dropping a gel electrolyte solution onto a conventional separator for lithium cell use and then ultraviolet-cross-linking the separator. Also, a similar fabrication of a nonwoven fabric-SPE composite is reported in the 8th International Meeting on Lithium Batteries, Extended Abstracts, pp. 284-285 (1996). Further, a similar fabrication of a separator-gel SPE composite is reported in J. Electrochem. Soc., 142, pp. 683-687 (1995).
According to Japanese Patent Application Laid-Open (kokai) No. 5-217416, a porous film-SPE composite is obtained by impregnating a porous film similar to a separator with a gel electrolyte solution, adding a polymerization initiator to the film, and thermosetting the film. According to Japanese Patent Application Laid-Open (kokai) No. 7-272759, a porous film-SPE composite is fabricated by subjecting a separator to corona discharge treatment, applying a polyvinyl carbonate solution onto the separator, and evaporating the solvent. However, the above-described conventional techniques involve the problems described below and thus are not put into practical use.
According to U.S. Pat. Nos. 5,102,752 and 5,378,558 mentioned previously, an electrolyte component is dissolved at 70.degree. C. to become an electrolyte solution, with which a nonwoven fabric is impregnated to become a composite electrolyte. However, upon being stored for a long period of time at a temperature of not less than 70.degree. C., the composite electrolyte involves the problem that the electrolyte may ooze; that is, the high-temperature characteristics of the composite electrolyte involve a problem. Also, according to Japanese Patent Application Laid-Open (kokai) No. 5-205515 mentioned previously, a polymerization initiator is used to combine a separator and an SPE component into a separator-SPE composite. However, this technique involves the fear that residual polymerization initiator may affect the storability of cells employing the separator-SPE composite.
According to J. Electrochem. Soc., 142, pp. 683-687 (1995); the 8th International Meeting on Lithium Batteries, Extended Abstracts, pp. 284-285 (1996); and Japanese Patent Application Laid-Open (kokai) No. 5-217416, all mentioned previously, an SPE component indispensably contains an acrylic monomer and is used with a polymerization initiator. However, it is pointed out that an ester linkage included in the acrylic monomer and residual polymerization initiator are electrochemically unstable. Also, according to Japanese Patent Application Laid-Open (kokai) No. 7-272759 mentioned previously, in order to fabricate a uniform SPE-separator composite, a separator previously undergoes corona discharge treatment and is then combined with a polyvinylene SPE. However, this technique involves the problem that the internal resistance of a cell employing the SPE-separator composite becomes high due to a thick membrane having a thickness of not less than 100 .mu.m.
As mentioned previously, the electric conductivity of a gel SPE is one order of magnitude lower than that of a liquid electrolyte. Also, in view of the interfacial resistance present in the interface between SPE and an electrode, the electric conductivity of a gel SPE is two orders of magnitude lower than that of a liquid electrolyte. Accordingly, the absolute resistance of SPE must be reduced by reducing the thickness of an SPE membrane, specifically to not greater than 25 .mu.m, which is equivalent to the thickness of separators currently used in lithium cells. Thus, an SPE membrane having a thickness of not less than 100 .mu.m has too high an absolute resistance to be put into practical use. Also, the above-described conventional techniques involve a drawback of insufficient adhesion between an electrically insulating material and a polymer component, since the electrically insulating material is generally composed of hydrophobic components having low polarity. As a result, the state of polymers contained in the obtained composite solid electrolyte becomes nonuniform, and a potential release of a polymer component is involved, resulting in a failure to obtain good electrolyte characteristics.